1. Field of the Invention
The present invention relates to a piezoelectric/electrostrictive ceramic composition capable of forming a piezoelectric/electrostrictive portion which exhibits excellent mechanical strength, rarely exhibits a dielectric breakdown even when a high electric field is applied, and exhibits excellent piezoelectric/electrostrictive characteristics, a piezoelectric/electrostrictive device including a piezoelectric/electrostrictive portion obtained using the piezoelectric/electrostrictive ceramic composition, and a method of producing a piezoelectric/electrostrictive device.
2. Description of Related Art
A piezoelectric/electrostrictive device has been known as a device of which the displacement can be controlled with an accuracy of submicrometers. Such a piezoelectric/electrostrictive device is used in various applications such as a piezoelectric pressure sensor, a probe moving mechanism of a scanning type tunnel microscope, a rectilinear guide mechanism in an ultraprecise processing apparatus, a hydraulic control servo valve, a head of a VTR apparatus, pixels of a flat-panel image display apparatus, an injector for a diesel engine, and a head of an inkjet printer.
A multilayer piezoelectric/electrostrictive device obtained by alternately stacking a sheet-formed piezoelectric/electrostrictive ceramic composition and an electrode is suitable for obtaining a large displacement and a high output. However, since a large internal stress occurs in the multilayer device due to a large displacement, microcracks are likely to occur. Therefore, the mechanical strength and the dielectric breakdown strength of the piezoelectric/electrostrictive device must be improved for sufficient driving durability. Particularly, piezoelectric/electrostrictive devices employed in new applications such as a high-viscosity liquid discharge device and a diesel engine injector is required to exhibit an extremely large displacement. However, when the piezoelectric/electrostrictive device is driven with a large displacement, microcracks easily occur in the piezoelectric/electrostrictive portion. Therefore, in order to ensure excellent driving durability under conditions in which a large displacement is required, the piezoelectric/electrostrictive portion must have a higher dielectric breakdown strength.
Various studies have also been conducted on a piezoelectric/electrostrictive ceramic composition which forms a piezoelectric/electrostrictive material. For example, a Pb(Mg1/3Nb2/3)O3—PbTiO3—PbZrO3 ternary solid solution system composition (hereinafter also called “PMN-PZT composition”) or a piezoelectric/electrostrictive ceramic composition obtained by replacing Pb in the composition with Sr, La, or the like has been disclosed (see JP-B-4417103 and JP-B-45-8145, for example). With such a composition, it is expected that a piezoelectric/electrostrictive device would be obtained in which a piezoelectric/electrostrictive material, which determines the piezoelectric/electrostrictive characteristics of the piezoelectric/electrostrictive device and therefore is the most important component, exhibits excellent piezoelectric/electrostrictive characteristics (piezoelectric constant d, for example).
In order to improve the piezoelectric characteristics by further increasing the density of piezoelectric/electrostrictive material, there is disclosed a piezoelectric device including a piezoelectric material formed of a ceramic composition in which 0.05 to 10 wt % of NiO is added to the PMN-PZT ceramic composition (see JP-A-2002-100819, for example),
However, although increased to some extent, the density of the piezoelectric material of the piezoelectric device disclosed in JP-A-2002-100819 is still insufficient, and thus, the dielectric breakdown strength is insufficient. Therefore, a dielectric breakdown easily occurs when a high electric field is applied.